Yes because Sealion is a ground pounding campaign, there are many more objects than in most other campaigns. We aimed in the test team to keep framerates above 30 fps on our machines, which was nearly 18 months ago now.
(In some missions, several hundred objects have been lovingly hand placed for your destructive enjoyment.)
You can try to adjust these graphic options first:
Grass OFF
Trees V LOW
Shadows OFF
H
PS we did a poll at one point about whether people would like a 'lo fi' version with fewer objects but there was not enough demand to justify it, then.