mxwbuaa
I think that phaseology means to make only the minimum amount of speed reduction necessary to maintain the distance separation. i.e do not instruct a faster following aircraft to reduce speed by 50knots when only a 20knot reduction will be required to stop it from catching up to a leading aircraft. There is no limit on the number of speed control instructions that an approach controller may issue - it is a common form of control used to keep aircraft separated.
It is not normal to ask an aircraft in the approach phase to increase speed, though this is sometimes done early in the approach phase (say, 30+ nm from the airport) to expedite the arrival of the first aircraft in an approach sequence. Typically the instruction may be something like, "Maintain 250kts IAS to 20NM XYZ DME" where XYZ DME is at the arrival airport. As the aircraft approaches the final approach it will be progressively reducing speed in accordance with the company's SOP (Standard Operating Practice). Speeds quoted by chillie are what are typically used.