Some pedals don't spin particularly freely from new. This is OK as the friction does not increase with load and is a tiny power loss. Often happens on MTB pedals to get a tight seal to keep rubbish out - after a short time rubbish in the bearing costs more than the seal in terms of power loss.
If the pedals used to spin freely, you possibly throwing away a few watts away as the lack of spin will be friction in the bearings and will get worse under load. Also consider that that friction is probably wearing the moving components faster than they would if well lubed.
Lubing them could be as simple as a drop of oil (wet chain lube) in between the bits that move, or a complete strip and grease. Personally I avoid the 'complete strip and grease' as I find by the time pedals need that I prefer to save the time and install new ones.
I would go with a drop or ten of oil (left overnight then wiped off) as it will probably help a little and is worth a crack.