ATtiny power consumption with clock prescaler
Chapter 21 of the ATtiny44 datasheet has a lot of information about consumption vs. frequency and supply voltage.

A casual look indicates that the lowest clock frequency will lead to the lowest total consumption.

Table 21-2 contains some information about how much additional consumption you add from the various modules depending on frequency, again indicating that lower speed will net you lower consumption.

Depending on how fast you need the computation to be done you might be able to get away with the 128kHz clock speed.

I haven't looked at the I2C and SPI modules in detail but it is possible that they will not operate correctly at such a low frequency so you might consider bumping up the speed before doing any comms using those peripherals.