I think clever people like Kim talked about something called "latency", but, being an ignorant luddite, I don't know what that is, and it hasn't bothered me anyway.
Literally the time taken for a message to get from one end of the connection to another.
With a baked-bean-tin telephone, the sound travels at the speed of sound in string. With an analogue electric telephone, the change in voltage travels down the wires at the speed of light
[1]. Once you start doing things digitally, it takes a little time to convert from analogue to digital and back at each end, but not enough that it becomes a problem.
What really slows things down is when you take that digital datastream, break it into convenient chunks, stick extra data on there to say where it's supposed to go and send it out on a network
[2] that operates more like a postal service than a series of tubes
[3]. And then stick it back together in the right order at the other end. Sometimes a packet might go the long way round. Or get lost. Or be duplicated. Or a load of them get backed up and all arrive at once, like buses. There are various coping strategies, mostly involving keeping a buffer of to-be-processed data at the receiving end to even out the flow. But that means the data that arrives on time has to wait in the buffer, which adds to the total journey time. (This applies to any kind of data, but it's only usually a problem with things that have to be responsive in real time like telephone calls, telnet/ssh sessions or video games. If a web page takes half a second to start loading, you probably won't notice.)
For voice/video, latency becomes noticeable when it's in the few-tenths-of-a-second range, and makes a conversation seem stilted.
Historically, cellular connections have had vastly more latency
[4] than wired broadband, but it seems to have greatly improved in the 4G era. You can still get inconsistent latency if you're using the connection while moving around, due to the vagaries of hand-over from one cell to the next.
That it hasn't been a problem is a testament to the performance of the network.
[1] In copper, which is a bit slower than the speed of light in a vacuum[5].
[2] Eg. The Internet.
[3] ©2006 Ted Stevens
[4] When I first got a mobile phone that could do such things, the round trip time could be of the order of seconds. You could type a command at a command line, and watch the characters appear one at a time a second or two later.
[5] Which can itself start to be a problem if your signal has to travel tens of thousands of miles, eg. out to a geostationary satellite and back (these days a sufficiency of undersea optical fibre means that most telecoms generally don't).
