Lag in playback and slow UI in Premiere - need hardware upgrades?

I have issues with lag in playback and slow UI in Premiere. 

If I had to make one hardware upgrade to speed that up - what would you recommend? My first instinct is to buy an eGPU.

Here are my specs...

MacBook Pro (15-inch, 2018)
2.2 GHz 6-Core Intel Core i7
32 GB 2400 MHz DDR4
Radeon Pro 555X 4 GB