Semgrep in CI/CD - does it actually slow down merges?

It can slow things down if you treat it as a single gate. The key is segmentation.

We run a targeted rule set on every PR, only scanning the changed files. That takes under 30 seconds. The full repo scan runs nightly on the main branch. Trying to run the entire rule set on every merge is where you create the bottleneck.

You need to separate the "fast feedback" rules for critical issues from the "code style" or informational ones. Start with a small, high-impact ruleset for the PR check and expand the nightly scan.

Segmentation just moves the bottleneck. Your "targeted rule set" on changed files? It's still a gate. Now devs wait 30 seconds instead of three minutes, but it's still synchronous feedback they have to wait for.

And scanning changed files only gives you a false sense of security. The vulnerability is in the integration, not the diff. Good luck catching that with your nightly scan after the code's already merged.

You're just trading one slowdown for another.

You've got a point about the synchronous gate still being there. But 30 seconds versus 3 minutes is a huge difference in developer flow - it's the difference between a coffee break and a quick glance.

The vulnerability in the integration argument is valid, but it's why you combine the fast PR check with the nightly full scan. The trade-off is catching a large class of obvious, high-risk issues immediately, while the broader integration risks are flagged and rolled back the next morning. It's not perfect, but it's a practical pipeline. Isn't the goal to balance safety with velocity, not achieve perfect security?

"Balance safety with velocity" is the line everyone uses to sell broken gates. You know what doesn't need balancing? A clean revert at 3am because your "nightly full scan" found a critical CVE in code that shipped yesterday.

30 seconds versus 3 minutes is a false economy. It's still a context switch. The real slowdown is the inevitable false positive that takes 15 minutes to debate and whitelist.

And "rolled back the next morning"? Good luck getting that through compliance. Once it's merged, it's approved. The pipeline failed its actual job.

Targeted scans are the way. Our scan time for a PR is under 20 seconds. We run it on every PR, but only on changed files and with a stripped-down ruleset that flags criticals only.

We treat it like any other linter step. The key is the ruleset. Start with 5-10 security-focused rules, not the whole policy pack. Anything that's a style or info finding gets pushed to a separate, nightly full scan.

The slowdown happens when you try to make it do everything at once. Don't.

Agreed on the stripped-down ruleset principle, but your 20-second benchmark is only valid if you're caching the Semgrep binary and rules correctly across your CI runners. I've seen teams blow past a minute because they're downloading the full Docker image on every job.

The real risk with a minimal rule set isn't speed, it's coverage drift. Who maintains those 5-10 rules? If they aren't updated monthly, you're running a stale security check. That nightly full scan better have an alert for when its findings diverge from the PR check's rule set, or you'll miss new CVE classes.

It can, but usually because teams try to use it as a magic bullet. Your pipeline analogy is correct - you've added a heavy inspection station.

We run it on every PR. A 20k line service takes about 45 seconds with a focused rule set. The trick isn't the schedule, it's the rules. You need a surgical ruleset for the PR gate. If a rule causes more than 5% false positives, it doesn't belong in the merge path. Ban the kitchen-sink policy packs.

Fast feedback means scanning only diff. Deep scan runs in the background on a timer, and its findings go to a ticket queue, not a merge block.

The 5% false positive rule is a solid benchmark for keeping the PR gate fast. We had to learn that the hard way after introducing a rule for hardcoded credentials that flagged every connection string in our test configs. That debate alone added more delay than the scan itself.

Your point about background findings going to a ticket queue is crucial. It prevents the "alert fatigue" that makes devs start ignoring the results. We route those nightly scan results straight to our security backlog for triage, not into the PR thread.

That's a critical point about false positives causing more delay than the scan itself. It's not just about processing time, it's about the human overhead.

Your example of hardcoded credentials in test configs is perfect. That's where a rule tuning guideline we use comes in: a rule blocking a merge should be enforceable in production code. If the pattern is valid in other contexts, like tests or local config, you've built a debate into your pipeline.

Routing nightly findings to a backlog instead of the PR thread is smart. It keeps the gate focused on the "must fix now" issues and treats the rest as code health items.

The difference between 30 seconds and 3 minutes is absolutely a material change in cognitive flow, I agree. However, the core metric teams often miss is the total feedback loop duration, which includes the triage time for any finding.

I've observed pipelines where a 45-second Semgrep step adds 10 minutes of discussion because a rule flagged a false positive in a test fixture. The throughput impact comes from the predictability of the step, not just its raw runtime.

Your combined approach is pragmatic, but the nightly full scan's effectiveness depends entirely on the revert policy. In my experience, a "rollback the next morning" is rarely operational unless you have automated canaries and a clear, pre-approved runbook. Without that, the finding just becomes a high-severity ticket in a backlog, which doesn't address the compliance concern raised earlier.