What exactly are low speed devices?
Low speed is somewhat subjective; in the case of I2C, the standard speed is up to 100kb/sec, full speed (previously known as high speed, confusingly) is 400kb/sec. These are the two most common speed ratings.

The packet size and payload sizes generally are not impacted by the speed definition, although most interfaces have 8 and 16 bit payloads, but may easily have more.

The "low speed interface" terminology was introduced (quite informally) to distinguish these interfaces from high speed interfaces (operating at several Mb/sec such as 10/100 ethernet or even Gb/sec rates such as Fibre channel, PCI Express, Infiniband and the fairly recent 100GE ethernet to name but a few).

Many high speed interfaces are serial in nature (there are advantages at the physical layer due to signal skew issues in parallel interfaces).

As I noted, there is not really a hard definition, but generally, interfaces running below perhaps 1Mb/sec are usually considered low speed.