I think it's a complaint of JetBrains IDEs loving to eat memory (and possibly having a large install size), which is true, although historically Eclipse, NetBeans and others have also had a similar reputation, with Eclipse also being modular to the point where some people's opinion of it has been soured due to bad tools built on top of it, as well as an arguably awkward workflow at times.

Then again, it's all probably relative - most IDEs (including the likes of Visual Studio) are slow compared to something like Lazarus, VS Code can feel slow compared to Sublime or Vim but whether that matters much is up to the reader. Personally, I like the features that JetBrains tools have, and those are more or less my daily drivers (I pay for the Ultimate package of all tools, alongside GitKraken, MobaXTerm and some other software), others might differ in that aspect.

That said, I don't think that JetBrains Fleet is quite as good as VS Code yet, so I use VSC for my more lightweight editing needs or on lower spec devices, sometimes also dropping down to Notepad++/Gedit or Nano for simple text/config files as well.