I try to play a 1080p h264 mkv movie on old laptop with Intel HD Graphics 5500.
On both MPC-BE and MPC-HC, with default EVR (custom presenter) renderer, the video lags randomly. When I change the renderer to MadVR, the video played smoothly. I'm actually surprised with that.

Is my laptop too weak for this simple video playback?

Just for lulz, I try to play 4k AVC1 video. It plays at worse than 1fps.

old processor cannot play newer format like vp9 or AV1 need to switch to CPu hence the lag. Intel HD Graphics 5500 supports hevc and h264 until 4K but I don't think it will be smooth on also
stay with H264 1080 should be no problem. Maybe ur setting issue cuz older hardware need to use hardware accelereration cannot run on CPU for video decoding it will get to slow and lag. Just make sure video format and player support the hardware decoding the coder of ur GPU then should be no issue
TLDR stick to h265 or h264 using hardware decoding 1080p still can play

MPC no longer in active development. Try using VLC instead.

Try PotPlayer...

If also cannot play then time to upgrade. Open the task manager to view the cpu/gpu utilisation. CPU aint really built for video decoding and most of them rely on the GPU to decode these.

the fork clsid2 and mpc-be are.

I have a same GPU here able to play 4K video smoothly. I use K-Lite Codec Pack (Mega) https://codecguide.com/download_kl.htm

For video decoding
first, weak GPU/CPU cant handle EVR, EVR use 2d or some sort of resampler that need some hardware resource.
but newer renderer is required for DXVA aka GPU hardware decoder, older video renderer (Overlay Mixer, VMR7) will not work with DXVA.
When selecting video renderer MPC will tell you which one is suitable for DXVA

Second, GPU now handles video decoding by default, if supported and presumed not buggy, else all CPU do hard work aka GPU hardware decoding not working, driver issue.
your CPU will max out if play AV1 in youtube, in 4k/8k

because the native video decoding capabilities are baked into GPU, if your gpu too old then new codec it will not be supported, no matter what codec you use, your CpU will do all the hard work.

TLDR version :
Old GPU = h264 only
not so old = h265
new = VP9 , AV1
future = h266/VVC