Why is average waiting time is the same for FIFO and LIFO
Maybe an example will help, consider lets say for 3 minutes we add 2 tasks each minute, and we process 1 task a minute. With a FIFO queue the timeline will be:


t(0): 'a' and 'b' added: [a, b] - 'a' processed after waiting $0$ - [b]
t(1): 'c' and 'd' added: [b, c, d] - 'b' processed after waiting $1$ - [c, d]
t(2): 'e' and 'f' added: [c, d, e, f] - 'c' processed after waiting $1$ - [d, e, f]
t(3): [d, e, f] - 'd' processed after waiting $2$ - [e, f]
t(4): [e, f] - 'e' processed after waiting $2$ - [f]
t(5): [f] - 'f' processed after waiting $3$ - []


Average wait time: $(0 + 1 + 1 + 2 + 2 + 3) / 6 = 1.5$, longest wait time: $3$

Then consider a LIFO stack:


t(0): 'a' and 'b' added: [a, b] - 'b' processed after waiting $0$ - [a]
t(1): 'c' and 'd' added: [a, c, d] - 'd' processed after waiting $0$ - [a, c]
t(2): 'e' and 'f' added: [a, c, e, f] - 'f' processed after waiting $0$ - [a, c, e]
t(3): [a, c, e] - 'e' processed after waiting $1$ - [a, c]
t(4): [a, c] - 'c' processed after waiting $3$ - [a]
t(5): [a] - 'a' processed after waiting $5$ - []


Average wait time: $(0 + 0 + 0 + 1 + 3 + 5) / 6 = 1.5$, longest wait time: $5$