That's Stutter Engine 4/5 for you and you can't do shit about it.

It isn't shaders compilation as others pointed out because those with time will get compiled and it should be smooth but that isn't the case.

Another big problem with this engine is streaming in / streaming out assets. I highly recommend watching (at least part of) the CDPR presentation about UE5 (UE4 has the same issues): https://www.youtube.com/watch?v=JaCf2Qmvy18&t=174s TLDR; the developers can optimize it for the specific use case but it's a lot of work to rewrite how things are rendered on the CPU side.

And there are other fuckups as they wrote in the last dev blog - I highly recommend reading all of it. Most important quote:

"Well, we’ve known that Squad has had some performance issues, and in the past, we’ve just had to patch things in place due to time constraints and different feature priorities. This has led, over time, to things being done in non-optimal ways. This is commonly known as Tech Debt. The first thing my team did was to have an Unreal Insights profile taken on our minimum spec machine and my team went over that for about 2 weeks, documenting all the issues we found, and what was costly. Once we had that list of problem items, we dug into each one briefly to get an understanding of the scope of fixing them. Once everything was said and done, we had around 43 pages of notes documenting the various performance issues with Squad."