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It depends on what you're trying to accomplish.
Pinging the gateway IP tells you little about performance. If fact, on some Cisco devices, they support a "special" responder to much more accurately report on access time to the device's interface vs. using a generic ping. (NB: the special responder also requires a special sender.)
You didn't describe what model your WAN router is. Even if your LAN switches are "dumb", maybe your WAN router isn't. However, if it too is "dumb", then if pinging between other LAN hosts is good, again, you likely don't have a LAN problem.
You may have a WAN congestion problem, most often caused by the amount of bandwidth to/from the Internet. Sometimes also caused by an "undersized" WAN router. (The two are not mutually exclusive.) Without a "smart" router, it will be difficult to analyze your actual situation.
You might trying pinging through the WAN router. If you can ping to an known outside host, that is relatively inactive, and doesn't have congestion on its side, high ping times to it may indicate WAN congestion.
If you do have WAN congestion, with a "smart" router, you can sometimes provide more predictable performance using QoS. Often to improve WAN performance, you'll need a higher bandwidth WAN link and/or more "powerful" WAN router.
