The Runtime Complexity (full) of the given CpxTRS could be proven to be BOUNDS(n^1, INF).

(0) Obligation:

Runtime Complexity TRS:
The TRS R consists of the following rules:

sum(0) → 0
sum(s(x)) → +(sqr(s(x)), sum(x))
sqr(x) → *(x, x)
sum(s(x)) → +(*(s(x), s(x)), sum(x))

Rewrite Strategy: FULL

(1) DecreasingLoopProof (EQUIVALENT transformation)

The following loop(s) give(s) rise to the lower bound Ω(n¹):
The rewrite sequence
sum(s(x)) →⁺ +(sqr(s(x)), sum(x))
gives rise to a decreasing loop by considering the right hand sides subterm at position [1].
The pumping substitution is [x / s(x)].
The result substitution is [ ].

(2) BOUNDS(n^1, INF)

(3) RenamingProof (EQUIVALENT transformation)

Renamed function symbols to avoid clashes with predefined symbol.

(4) Obligation:

Runtime Complexity Relative TRS:
The TRS R consists of the following rules:

sum(0') → 0'
sum(s(x)) → +'(sqr(s(x)), sum(x))
sqr(x) → *'(x, x)
sum(s(x)) → +'(*'(s(x), s(x)), sum(x))

S is empty.
Rewrite Strategy: FULL

(5) TypeInferenceProof (BOTH BOUNDS(ID, ID) transformation)

Infered types.

(6) Obligation:

TRS:
Rules:
sum(0') → 0'
sum(s(x)) → +'(sqr(s(x)), sum(x))
sqr(x) → *'(x, x)
sum(s(x)) → +'(*'(s(x), s(x)), sum(x))

Types:
sum :: 0':s:+' → 0':s:+'
0' :: 0':s:+'
s :: 0':s:+' → 0':s:+'
+' :: *' → 0':s:+' → 0':s:+'
sqr :: 0':s:+' → *'
*' :: 0':s:+' → 0':s:+' → *'
hole_0':s:+'1_0 :: 0':s:+'
hole_*'2_0 :: *'
gen_0':s:+'3_0 :: Nat → 0':s:+'

(7) OrderProof (LOWER BOUND(ID) transformation)

Heuristically decided to analyse the following defined symbols:
sum

(8) Obligation:

TRS:
Rules:
sum(0') → 0'
sum(s(x)) → +'(sqr(s(x)), sum(x))
sqr(x) → *'(x, x)
sum(s(x)) → +'(*'(s(x), s(x)), sum(x))

Types:
sum :: 0':s:+' → 0':s:+'
0' :: 0':s:+'
s :: 0':s:+' → 0':s:+'
+' :: *' → 0':s:+' → 0':s:+'
sqr :: 0':s:+' → *'
*' :: 0':s:+' → 0':s:+' → *'
hole_0':s:+'1_0 :: 0':s:+'
hole_*'2_0 :: *'
gen_0':s:+'3_0 :: Nat → 0':s:+'

Generator Equations:
gen_0':s:+'3_0(0) ⇔ 0'
gen_0':s:+'3_0(+(x, 1)) ⇔ s(gen_0':s:+'3_0(x))

The following defined symbols remain to be analysed:
sum

(9) NoRewriteLemmaProof (LOWER BOUND(ID) transformation)

Could not prove a rewrite lemma for the defined symbol sum.

(10) Obligation:

TRS:
Rules:
sum(0') → 0'
sum(s(x)) → +'(sqr(s(x)), sum(x))
sqr(x) → *'(x, x)
sum(s(x)) → +'(*'(s(x), s(x)), sum(x))

Types:
sum :: 0':s:+' → 0':s:+'
0' :: 0':s:+'
s :: 0':s:+' → 0':s:+'
+' :: *' → 0':s:+' → 0':s:+'
sqr :: 0':s:+' → *'
*' :: 0':s:+' → 0':s:+' → *'
hole_0':s:+'1_0 :: 0':s:+'
hole_*'2_0 :: *'
gen_0':s:+'3_0 :: Nat → 0':s:+'

Generator Equations:
gen_0':s:+'3_0(0) ⇔ 0'
gen_0':s:+'3_0(+(x, 1)) ⇔ s(gen_0':s:+'3_0(x))

No more defined symbols left to analyse.