For a project on a minecraft server I used the new /forceload command to keep some chunks loaded. There is probably about 10 chunks not counting the spawn chunks that are forceloaded. I've also noticed that the server's memory usage has gone up as well as the "Running behind" messages. There has also been an increase in players so that could also be the issue. In my mind it's not that much different than having another player on, but does it put a bit more stress on the server because they are in various location and not grouped together (more like 10 players instead)? This is in 1.13.2 minecraft with vanilla server.
Yes, due to bug MC-141482
/forceload loads nine chunks for every chunk you mark. This does indeed mean that there are way more chunks loaded than you would expect, especially if they are spread out.
Other than that, you probably have stuff happening in those chunks. Usually that matters more than the pure number of chunks. For example turning off redstone lines lags the server a lot, as well as putting colliding entities into one spot and moving blocks high in the air, just to name a few examples.
If you're already getting "Can't keep up" messages, then the server is overloaded and you should consider doing something against that, like running it on a more powerful device, setting the render distance lower or optimising the always loaded areas against lag.
"A lot" may be an overestimate. By default, the server operates chunks within radius of 10 taxicab distance (for a total of 21x21 or 441 chunks) src around the player. So
/forceload set to 19x19 area effectively ( MC-141482 ) puts the load equivalent of one lone player, minus hostile mob spawning and some other burdens that happen only in the actual player presence. If you load a dozen chunks or so (
/forceload'ing 2 chunks will load 12...) it's not such a huge problem.
Obviously what impact it has heavily depends on what happens in these chunks. Minecarts with hoppers create a lot of lag (even when immobile, e.g. stuck into the ground of your farms over standard hoppers). Mobs like villagers and golems cause significant lag. Redstone contraptions, especially with no light update protection can cause lag. Single observers are not a problem, but filling large volume with these (sugarcane or pumpkin farms) can become a burden. OTOH random tick based farms with water transport, like cactus, are pretty lightweight, taking up RAM but little of CPU time.
Also watch out for accidentally loading more chunks than you need. For example, a hopper with an item inside aimed from a loaded chunk into unloaded chunk loads it, even if there's nothing to receive that item (and if there are other hoppers aimed into other chunks, these get loaded too - so 'pipelines' of hoppers can load a bunch of chunks you didn't intend to load. OTOH this may be a good way to expand your chunkloaded area without creating excessive borders. Forceload 1 central chunk of a 48x48 farm, and append a collection area of 1 chunk through loading it with a hopper pointed into it from one of the border chunks, for a total of 10 instead of 12.