IntelliJ 2020.3: Dart code completion beachballs Mac

Hi,

I use IntelliJ IDEA 2020.3 (Ultimate Edition), Build #IU-203.5981.155, built on November 30, 2020. It runs with the JetBrains JDK 11.0.9.1+11-b1207.1 x86_64. My Mac runs Big Sur 11.1.

I work about six hours a day on a Flutter project. Flutter uses Dart. I got both plug-ins in the latest versions installed: Flutter 52.1.5, Dart 203.6912.

For the last two-three weeks, IntelliJ slows down so much that it's unusable about once a day. Code completion beachballs for 15 seconds before showing an empty window. Even the preferences turn up an empty window. The slowness spreads to other applications, like Safari. It didn't use to do this. Now during all this time, IntelliJ doesn't use 100% of a CPU core, from what I can see.

I'm not sure if IntelliJ IDEA 2020.2 had the same problem. Does anybody know how to fix this?

Here's a video demonstrating it:

• IntelliJ is on the right, Activitiy monitor on the left. SnagitRecorder records the screen. I got multiple displays.
• I start out with IntelliJ already beachballing.
• After 12 seconds, I get control back.
• At about 15 seconds, I delete an attribute and hit [Alt]-[Space] to invoke code completion.
• IntelliJ beachballs, but you only see it around 20 seconds when I move the cursor back onto the main screen.
• At 30 seconds, IntelliJ shows an empty code completion dialogue. It then continues to beachball.
• At 46 seconds, IntelliJ shows a second empty window (hover help?).
• I move the cursor. Both empty windows disappear.
• At 50 seconds, I select the IntelliJ preferences. IntelliJ beachballs.
• At 66 seconds, an empty preferences window shows. I close it.

Regards,
Karsten Silz