In most cases attaching a slower device to a switch will not slow down communication over other ports. So you can expect the gigabit capable devices on the gigabit switch to be able to communicate with each other at gigabit speeds.
However there are some cases where bottlenecks can behave differently from what users would expect. That is due to a concept known as back pressure.
When an outgoing port on a switch experiences congestion, the switch can either drop packets or tell the sender to slow down.
If the switch responds to congestion by dropping packets, then TCP congestion control will kick in, and it can be difficult to utilize the full gigabit speed across a LAN. For this reason some vendors have decided to tell the sender to slow down instead. That works great for sending a single TCP stream across a LAN.
But once multiple flows share a link, back pressure can have undesired side effects.
If one of the gigabit capable devices is sending data simultaneously to two devices where one is able to receive at gigabit speeds and the other can only receive at 100Mbit/s, the switch will notice that the 100Mbit/s link cannot keep up and tell the sender to slow down. When the sender slows down, it affects both flows. So instead of sending 900Mbit/s to one receiver and 100Mbit/s to the other, the sender may end up sending only 100Mbit/s to each receiver.
If you link multiple switches together and have a mixture of different speeds, such problems are more likely to occur. But in principle it can happen even if you have just a single switch with all links running at identical speeds, it just takes the right combination of flows to trigger it.
In my experience these problems are very rare though. I have only seen them while stress testing networks. I haven't experienced them during normal usage.